A high-sensitivity atomic magnetometer using spin of alkali metal gas or rare gas has been proposed. U.S. Pat. No. 7,038,450 has proposed an atomic magnetometer in which a cell including an alkali metal vapor is irradiated with linearly polarized light (beam) from a linearly polarized light source for a pump beam and with circularly polarized light (beam) from a circularly polarized light source for a probe beam. Further, in order to measure a polarization rotation angle of the probe beam, a photo-diode array is used as a photo-detector.
Incidentally, information on a magnetic field intensity (strength) depending on a measurement position is measured as magnetic field gradient information in some cases. In such cases, differential information of the magnetic field intensity (magnetic field gradient information) is obtained by using differential information on electric signals obtained by being detected through a photo-detector provided correspondingly to each of measurement positions.
However, actually, it is considered that a condition concerning an amplification factor or noise of an individual photo-detector per se is not strictly uniformized. That is, when the magnetic field gradient information is intended to be obtained by using the electric signals obtained from the photo-detector, there is liability to mixing of noise, on the basis of characteristic difference between individual photo-detectors, in the magnetic field gradient information.